Burner

ABSTRACT

The invention is directed to a burner comprising (a) a burner pipe, (b) mixing means arranged in the burner pipe for mixing an oxygen-containing gas and a fuel gas and for discharging the resulting gas mixture within the burner pipe; and (c) stabilising means downstream of the mixing means in the centre of the burner pipe, wherein the stabilising means comprises a conically shaped body which widens in flow direction.

The present invention relates to a burner, more specifically a gas burner wherein a fuel gas and oxygen-containing gas are pre-mixed before ignition. The burner of the present invention is particularly suitable as an auxiliary burner or heat-up burner in partial oxidation reactors for the manufacture of synthesis gas by partial oxidation of a gaseous hydrocarbon-containing fuel.

Partial oxidation processes are, for example, used for the production of pressurized synthesis gas, fuel gas or reducing gas. To this end, gaseous or liquid hydrocarbon fuels and oxygen-containing gas are introduced into a reactor via a main gasification burner as for example described in WO-A-96/03345. In the reactor a flame is maintained in which the fuel reacts with the oxygen-containing gas to form a gas comprising carbon monoxide and hydrogen.

To start-up a partial oxidation reactor, the temperature of the reactor and its refractory lining should be raised to a level sufficiently high to enable the fuel to self-ignite with the oxygen-containing gas, so that the partial oxidation reaction is initiated. To this end an auxiliary burner can be used. The auxiliary burner must have sufficiently high capacity to increase the temperature of the reactor and refractory lining to such high level that the mixture of oxygen-containing gas and hydrocarbon fuel spontaneously ignites when these reactants are introduced via the main gasification burner into the reactor. It is, therefore, always an aim to maximize the capacity—i.e. the power produced per area unit at the burner mouth—of the auxiliary burner.

The auxiliary burner is suitably positioned, either permanently or in a retractable manner, inside the main gasification burner along its central axis. In this way there is no need for a separate nozzle and refractory plug in the reactor. Because the dimensions of the main gasification burner are limited the auxiliary burner must be compact to fit within said main gasification burner.

For example, EP-A-2,085,696 discloses an auxiliary burner comprising an igniter burner connected to a first fuel supply line and one or more secondary fuel supply lines each leading to a burner mouth at a distance downstream the igniter burner. A feed for a flow of an oxygen-containing gas is positioned upstream of the igniter burner. The burner mouths of the secondary fuel supply lines are each suitably surrounded by a conical disk widening in the flow direction. Nozzle openings in the burner mouth are directed to guide the gas flow along the interior side of the conical disk. This results in improved stability of the flames coming from the burner mouths of the secondary fuel supply lines.

The flame coming from the auxiliary burner according to EP-A-2,085,696 consists of multiple flames which makes it more difficult to control the flame stability. Furthermore, in the auxiliary burner of EP-A-2,085,696 no premixing of fuel and oxygen-containing gas takes place, which increases the risk of soot formation inside the burner. Finally, the manufacture of this auxiliary burner is relatively complex because of the multiple fuel supply lines which should be symmetrically arranged around the central igniter burner.

The present invention aims to provide an improved burner.

Accordingly, the present invention relates to a burner comprising

-   (a) a burner pipe, -   (b) mixing means arranged in the burner pipe for mixing an     oxygen-containing gas and a fuel gas and for discharging the     resulting gas mixture within the burner pipe; and -   (c) stabilising means downstream of the mixing means in the centre     of the burner pipe,     wherein the stabilising means comprises a conically shaped body     which widens in flow direction.

The burner of the present invention has a high heating capacity and is very compact. Furthermore, it has a single fuel supply line and is less complex to manufacture. It was also found that because of the specific configuration of the stabiliser the flame stability is very good, whilst at the same time a high throughput of fuel can be achieved. The compactness, high heating capacity, good flame stability and high fuel throughput of the present burner make the burner of the present invention very suitable as an auxiliary burner in the main burner of a partial oxidation reactor.

The expressions “flow direction”, “upstream” and “downstream” as used herein refer to the flow direction of the gases (i.e. the fuel gas, the oxygen-containing gas and the mixture of fuel gas and oxygen containing gas) in the burner when the burner is in operation.

The stabilising means or stabiliser is disposed in the centre of the burner pipe, along its longitudinal axis. The function of the stabilising means is to ensure a stable and homogeneous flow of the mixture of fuel gas and oxygen-containing gas in the zone between the stabilising means and the inner wall of the burner pipe, so that there is a stable flame downstream of the stabilising means towards the burner head.

According to the present invention the stabilising means comprises a conically shaped body at the downstream end of the stabilising means, with the conically shaped body widening in flow direction. Given the position of the stabilising means the axis of the conically shaped stabilising body coincides with the longitudinal axis of the burner pipe. The term “axis” as used in this connection in relation to the conically shaped stabiliser body refers to the straight line passing through the centre of the circular base and the (virtual) apex of the conically shaped stabiliser body, about which the lateral or outer surface of the conically shaped stabilising body has a rotational symmetry.

In addition to the conically shaped body the stabilising means may also comprise one or more cylindrically shaped bodies connected to the upstream end and/or downstream end of the conically shaped body, which cylindrically shaped body or bodies are connected with the conically shaped body in such a way that the central longitudinal axis of the cylindrically shaped part(s) coincide(s) with the axis of the conically shaped part. Any cylindrically shaped part of the stabiliser connected to the downstream end of the conically shaped part will suitably have substantially the same diameter as the circular base of the conically shaped part. In a preferred embodiment of the present invention the stabilising means further comprises a cylindrically shaped body connected to the upstream end of the conically shaped body. The downstream circular end of the stabiliser, i.e. the base of the conically shaped part or the downstream end of a cylindrical part of the stabiliser can be a closed circular plane or can be open. Accordingly, the conically shaped part and/or any cylindrically shaped part connected therewith at the upstream or downstream end can be massive or hollow. In one preferred embodiment the conically shaped body is a massive body. In another preferred embodiment the conically shaped body is a hollow body. In yet a further preferred embodiment the stabiliser consists of a conical part connected at its upstream end with a cylindrical part, with the conical part and cylindrical part both being hollow with the cylindrical part being closed at its upstream end.

The cone angle α is the angle between the axis and the lateral surface of the conically shaped stabiliser body. This cone angle a suitably is within the range of 5 to 45 degrees, preferably between 8 and 30 degrees, more preferably between 10 and 20 degrees.

Dimensions of the stabilising means may vary depending on the dimensions of the burner pipe. It has been found that the ratio between the diameter of the circular base of the conically shaped body of the stabiliser and the inner diameter of the burner pipe at this circular base is suitably in the range from 0.35 to 0.95, preferably 0.45 to 0.92 and more preferably 0.50 to 0.90. In case of a stabiliser comprising a conically shaped part and a cylindrically shaped part upstream of the conically shaped part the diameter of the cylindrical part will typically be 40 to 90%, preferably 45 to 85% and more preferably 50 to 80% of the diameter of the circular base of the conically shaped part. The ratio between the lengths of the various parts of the stabiliser will depend inter alia on the required duty and the gas velocity of the mixture of the oxygen-containing gas and hydrocarbon fuel gas and it is well within the normal competence of those skilled in the art to select the appropriate ratio for the anticipated circumstances.

The absolute dimensions of the burner pipe and stabiliser will, inter alia, depend on the intended use, gas velocities and required duty of the burner and it is well within the normal competence of those skilled in the art to select the appropriate dimensions. For example, the diameter of the circular base of the conically shaped body of the stabiliser may suitably be between 25 and 80 mm, whilst the diameter of the burner pipe at the place where the aforesaid circular base is arranged could suitably be anywhere between 1.1 and 2 times the diameter of the circular base.

A commonly experienced phenomenon in burners is resonance of the gases moving at high velocities through the burner. When the burner is in operation, such resonance could, for example, result in the burner making loud noises and/or the burner starting to vibrate. Depending on where the burner is operated, the sound intensity must be kept within acceptable limits. If the noise becomes unacceptably loud, that is, the sound intensity is too high, then measures must be taken to reduce the noise. If vibrations become too violent, then this may cause serious damage to the burner and will impact the duty and lifetime of the burner. Furthermore, it may even lead to damage to the main burner. In order to prevent or anyhow suppress any resonance effects, the burner of the present invention may also comprise one or more resonance-suppressing means, such as baffles, vanes or blades. These resonance-suppressing means will typically be arranged upstream of the conically shaped part of the stabiliser, for example on the cylindrical part connected to the conically shaped part at its upstream end.

The burner pipe suitably is a tubular body with a longitudinal axis, for example an open cylinder. The burner mouth, i.e. that end of the burner pipe where the flame exits the burner, may have the same diameter as the upstream part of the burner pipe, but could also have a smaller or larger diameter. The burner pipe could also widen in downstream direction starting at the point where the conically shaped part of the stabiliser starts, widening, for example, in parallel with the conically shaped part of the stabiliser. The skilled person will be able to select the optimum shape of the burner pipe depending on, inter alia, intended use, desired gas throughput, gas velocities and required capacity.

The materials of which the various components of the burner are made are those normally used for burners and it is well within the normal competence of those skilled in the art to select the appropriate materials depending on the maximum allowable temperatures which can be reached within the burner when in operation. An example of a commonly used material is heat-resistant steel.

The burner according to the present inventions is a burner of the pre-mix type. Hence, it comprises mixing means for mixing an oxygen-containing gas and a fuel gas upstream of the stabiliser, where the oxygen-containing gas and the fuel gas are mixed before being led around the stabiliser and ignited. Other types of burners are burners where the fuel gas and oxygen-containing gas are only mixed near the burner opening, where the resulting mixture is subsequently ignited. An example of this type of burner is disclosed in the aforementioned EP-A-2,085,696.

In the burner according to the present invention the mixing means suitably is a mixing tube having one or more inlets for the supply of fuel gas, one or more inlets for the supply of an oxygen-containing gas and one or more outlets for discharging the mixture of fuel gas and oxidising gas from the mixing tube in the space between the mixing tube and the inner wall of the burner pipe. The mixing tube is suitably arranged such that its longitudinal axis coincides with the longitudinal axis of the burner pipe. In one preferred embodiment of the present invention the mixing tube has a single inlet for fuel gas and multiple inlets for the oxygen-containing gas downstream of the inlet for fuel gas. The fuel gas can, for example, be injected through a nozzle at the upstream end of the mixing tube, whilst the oxygen-containing gas can enter the mixing tube through one or more holes or slits in the mixing tube. Typically the number of holes or slits will to some extent be determined by the dimensions of the mixing tube and by the amount of oxygen needed. Practically, the mixing tube may comprise from 1 to 10 holes, slits or differently shaped inlets for the oxygen-containing gas. If the fuel gas is blown in at high velocity, it will suck in the oxygen-containing gas and in the resulting turbulence inside the mixing tube both gases are intimately mixed. Alternatively, both oxygen-containing gas and fuel gas are injected into the mixing tube via suitable nozzles and in the resulting turbulence both gases are mixed. Any turbulence within the mixing tube may be enhanced by specific internals such as vanes.

The resulting mixture of oxygen-containing gas and fuel gas is then suitably released from the mixing pipe through multiple outlets downstream of the inlets for the oxygen-containing gas. Obviously the distance between the inlets for the oxygen-containing gas and the outlets of the mixture of oxygen-contain gas and fuel gas should be sufficiently long to allow effective mixing. Furthermore, whilst the gas mixture flows towards the stabiliser to be ignited, it also acts as a cooling means for the burner and its internal components. Cooling the burner and its components is important to keep the burner in operation. Too high temperatures will lead to deterioration of the components and ultimately to reduced heat capacity and poorer performance.

The oxygen-containing gas can for example be ambient air or oxygen-enriched air, i.e., air containing more than 21 vol. % oxygen.

The fuel gas to be used is a hydrocarbon-containing, combustible gas, suitably a hydrocarbon-containing gas that can be used as a feed to a partial oxidation reaction to form a mixture of carbon monoxide and hydrogen. Examples of suitable feeds are methane comprising feeds, such as natural gas, associated gas or a mixture of C₁₋₄ hydrocarbons. Preferably the feed comprises mainly, i.e. more than 80 vol. %, especially more than 90 vol. %, C₁₋₄ hydrocarbons. Very suitably natural gas or the effluent of a pre-reforming reactor, where natural gas is pre-reformed for a subsequent partial oxidation reaction to form a mixture of carbon monoxide and hydrogen, is used.

Preferably, the burner comprises igniting means such as a spark plug, a hot wire, or combinations thereof, to ignite the combustible mixture of fuel gas and oxygen-containing gas. The igniting means are suitably built-in within the burner at such location that the combustible gas mixture of oxygen-containing gas and fuel gas ignites towards the burner mouth, so that a stable flame is obtained. In a preferred embodiment of the present invention the igniting means are one or more—usually not more than four—spark plugs attached to the stabiliser. If the stabiliser comprises an upstream cylindrical part, then the spark plug or spark plugs are suitably disposed on this cylindrical part.

The burner is also suitably provided with a flame detection means. Such flame detection means will be arranged at such position in the burner pipe that it can effectively detect the flame. Typically this will be in, on or downstream of the stabiliser. Flame detection can for example take place by scanning ionization or UV. Preferably the flame detection means is an ionisation rod extending in flow direction from the centre of the circular base at the downstream end of stabiliser body. UV detection means, such as a flame eye, can also be suitably used, but as this type of detection means is less heat resistant, it should be located at such position that it is not exposed to the maximum heat of the flame. For example, UV detection means would be suitably arranged inside the stabiliser in case of a hollow stabiliser.

The burner of the present invention may further comprise parts and components which are normally present in burners of this type, such as gaskets, housings, flanges, rims, pipes for the supply of fuel gas and oxygen-containing gas, nozzles and the like.

The burner according to the invention is particularly useful as a high duty, compact auxiliary burner or heat-up burner for a partial oxidation reactor. Optionally, the burner can be an integral part of such a main gasification burner, for example, as a retractable part or as a fixed part. If the burner according to the present invention is used as an auxiliary burner and forms an integral part of the main burner, then the oxygen-containing gas for the main burner flows around this auxiliary burner. In this way this flow of oxygen-containing gas also serves as a cooling means for the outer wall of the burner pipe of the auxiliary burner. An additional or alternative way of cooling the burner pipe is by means of a closed-loop burner cooling water system. Suitably the burner of the present invention is designed such that the direct contact between the flame and the burner mouth is minimised, to avoid burning of and coke formation on the inside of said burner mouth.

The invention will now be described in more detail, by way of example only, with reference to the accompanying figure, wherein:

FIG. 1—shows the downstream part of a burner according to the invention in longitudinal cross section.

In FIG. 1 a stabiliser (2) consisting of a conically shaped part (3) and a cylindrically shaped part (4) connected to the upstream end of the conically shaped part (3) is disposed in the centre of burner pipe (1) along the longitudinal axis (7) of the burner pipe (1). The arrows indicate the flow direction of the combustible mixture of oxygen-containing gas and fuel gas, which mixture is obtained by mixing a fuel gas and oxygen-containing gas in mixing means arranged in an upstream section of the burner pipe. The cylindrically shaped part (4) of the stabiliser contains spark plugs (5), whilst an ionisation rod (6) extends from the circular base of stabiliser (2) at its downstream end. The central pipe (8) is used to support the stabiliser (2) and to guide the electric connection to the ionisation rod (6).

EXAMPLE

A burner having a downstream part in accordance with FIG. 1 of the following dimensions:

-   -   inner diameter of pipe (1) is 54 mm,     -   diameter of the circular base of the massive conically shaped         part (3) is 46 mm         was operated at an air to gas ratio of 17.2 with an air flow of         688 kg/hr and a natural gas flow of 40 kg/hr. The burner gave a         strong ionization signal of 19.9 mA demonstrating an excellent         flame stability. 

1. A burner comprising: (a) a burner pipe, (b) a mixer arranged in the burner pipe for mixing an oxygen-containing gas and a fuel gas and for discharging a resulting gas mixture within the burner pipe; and (c) stabilising means a stabilizer downstream of the mixer in the centre of the burner pipe, wherein the stabilizer comprises a conically shaped body which widens in flow direction.
 2. A burner according to claim 1, wherein the conically shaped body is a massive body.
 3. A burner according to claim 1, wherein the conically shaped body is a hollow body.
 4. A burner according to claim 1, wherein the stabilizer further comprises a cylindrically shaped body connected to the upstream end of the conically shaped body.
 5. A burner according to claim 1, wherein the cone angle of the conically shaped body is in the range from 5 to 45 degrees.
 6. A burner according to claim 1, wherein the ratio between the diameter of the circular base of the conically shaped body and the inner diameter of the burner pipe at this circular base is in the range from 0.35 to 0.95.
 7. A burner according to claim 1, wherein the mixer is a mixing tube arranged such that its longitudinal axis coincides with the longitudinal axis of the burner pipe.
 8. A burner according to claim 1, wherein the burner further comprises a flame detector.
 9. A burner according to claim 1, wherein the burner further comprises an igniter. 